workqueue: Adjust WQ_MAX_ACTIVE from 512 to 2048

WQ_MAX_ACTIVE is currently set to 512, which was established approximately
15 yeas ago. However, with the significant increase in machine sizes and
capabilities, the previous limit of 256 concurrent tasks is no longer
sufficient. Therefore, we propose to increase WQ_MAX_ACTIVE to 2048.
and WQ_DFL_ACTIVE is 1024 now.

Signed-off-by: Chen Ridong <chenridong@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>